The biggest difference in terms of the penalties is when it hits. Outside of gains during your turn, which are relatively rare for $5s, you cannot just Gship away the bonus cards from when you buy them. Obviously this changes if we include Outpost, Mission, Possession or just have 3 players.
In practical terms, this means that you rarely see more than a single card or two of bonus draw at any given time. $15/3buys is pretty close to not needing more cities and your deck is likely pretty close to auto-drawing when your opponent gets there. This extra card is mostly going to:
A. Push you up a $1 in buying power. This can make Lost city a bit of a trap card if I get more $5s even if they are slightly less powerful than an activated city. My buys may be less powerful (say Bazaar), but I will get more of them (particularly if we are pairing them with other $5/6 buys like say Rabble or Hunting grounds).
B. Increase your mid-game reliability. Sure some setups, like Vault/Kc might have a big impact with one extra card, but generally once you are hitting $5 reliably the next price point to matter is $7 or $8 with 2 buys. Say you are fishing for one enabling card (like Village or Smithy) in 10, giving your opponents an extra card swaps them from 50% odds of hitting off the engine to something around 60%. I have not a found a good way to track this, but at some point that extra card will give the a significant leg up on getting multiple cards a turn from drawing half the deck. On the other hand, once he is drawing his whole deck, who cares how many he started with in hand (beyond some obvious Storeroom type setups).
Another pronounced effect of the penalty is with combos. Tr/Witch is one of those things that may make playing the odds bad, gaining 20% of an extra curse & being half a hand closer to a reshuffle is starting to be a big deal. Worse Tmap/Tmap & Urchin/Urchin you can see a very large increase in you. The penalty hurts you a lot more when your opponent is going for something that is not a draw engine that needs to line things up (e.g. Chapel/Bishop), preferably quickly
In general, I think Lost city is one of those rare draw cards that works best with other terminal draw. You can very quickly get through your deck, particularly if the other guy is buying the penalty for you and then start mixing in cards to ramp up sooner (or use them more aggressively to ramp up buying power sooner).